1. Field of the Invention
The present invention relates to an irradiation apparatus having a movable irradiation head.
2. Description of Prior Art
There have been known various irradiation apparatuses, such as radiotherapy apparatuses and X-ray projectors, having a movable irradiation head for emitting or projecting a beam of radiation. A typical type of such irradiation apparatus has a support structure for the irradiation head, which enables a wide range of controlling of the orientation of the axis of the radiation beam emitted from the irradiation head in two-axes control. The support structure comprises a fixed base adapted to be fixed on an installation site such as a floor of a treatment room, a rotary support carried by the base for rotation about Y-axis extending horizontally, and a head support carried by the rotary support for rotation about X-axis which is fixed relative to the rotary support and extends in perpendicular to the Y-axis. The irradiation head is fixed to the head support.
In this geometry of the support structure, the intersection between the X- and Y-axes is fixed and immovable relative to the floor on which the irradiation apparatus is fixedly installed, irrespective of the rotational positions of the rotary support and the head support. Further, the irradiation head is mounted on the head support such that the axis of the radiation beam from the head passes through the intersection between the X- and Y-axes, thereby it is ensured that the radiation beam always passes through the fixed point defined by the intersection of the axes. Generally, the irradiation field in the body of a patient is positioned on this fixed point during inspection or treatment, in order to achieve the positional precision of irradiation. The fixed point, i.e. the intersection between the X- and Y-axes, is commonly referred to as "isocenter." Z-axis can be defined as one extending in perpendicular to both the X- and Y-axes and passing through the isocenter.
In general, it is desired that any moving parts should be balanced as a whole whatever positions they have been moved. For the irradiation apparatus described above, the balanced state can be achieved when the center of gravity of the head support with any parts supported thereby lies on X-axis and the center of gravity of the rotary support with any parts supported thereby lies on Y-axis.
However, because the irradiation head is typically far heavier than any other components and parts of the irradiation apparatus, an excessive imbalance tends to occur if no balancing mechanism is provided. Any excessive imbalance may i) cause an undesirable deformation of parts resulting in an inaccurate positioning of the radiation beam, ii) require drive motors and gear trains for controlling the positions of moving parts to have great capacities, and iii) allow a "free-fall" if a power failure happens and the drive motors are deenergized (the free-fall is the movement of a part or parts of the apparatus induced by the gravity, tending to lower the center of gravity of the moving parts as a whole).
There has been proposed no irradiation apparatus which is of the described type and has a balancing mechanism that provides sufficient results with simple and reliable structure.